(a) Field of the Invention
The present invention relates to a telephoto lens system having a large aperture ratio and a rear focusing system (a system fo focusing by shifting a rear lens group) comprising a floating means (shifting specific lens for correcting aberration independently of other lenses in a lens system).
(b) Description of the Prior Art
Recently, large aperture ratios such as F 2.0 or F 2.8 and high performance as well as shortening of minimum photographing distances are desired even for telephoto lens systems having focal lengths on the order of 200 mm to 300 mm. Further, lens systems which are light in weight and assure high operability for focusing are desired.
As a prior example designed to meet the desire, there exists a lens system which comprises lenses made of a glass material having an especially low dispersing power (very large Abbe's number) and adopts an internal focusing system which performs focusing by shifting certain lens(es) in the lens system. The above-mentioned internal focusing system can be contrived in various types. As lens systems adopting known internal focusing systems, there exist those disclosed by Japanese unexamined published patent No. 139732/50 (prior example No. 1), Japanese unexamined published patent No. 134425/53 (prior example No. 2) and Japanese unexamined published patent No. 17723/49 (prior example No. 3). Among these lens systems, prior example No. 1 is a lens system in which only one lens component is movable in its rear lens group. Prior example No. 2 is a lens system in which two or more lens components are movable with relative airspaces reserved therebetween kept unchanged. Prior example No. 3 is a lens system comprising a floating means in which two lens components are movable, relative airspace between said lens components is variable and one of said lens components has an aberration correcting function.
These prior examples adopting the internal focusing systems have satisfactory operability since the lens components to be shifted for focusing have low weights and are displaced for short distances. Further, these prior examples made it possible to focus the lens systems on an object located at short distance at which photographing magnification is apploximately 1/10. However, these lens systems adopting the internal focusing systems have a common drawback that they allow remarkable variations of aberrations in focusing, thereby making it difficult to maintain stable performance free from variations of aberrations within a range from infinite distance to short distance, and provide deteriorated images when focused on an object at a short distance though they provide satisfactory images when focused at infinite distance. Out of the above-mentioned examples, prior example No. 1 allows remarkable variations of aberrations when the movable lens component having aberration by itself is shifted for focusing. Prior example No. 2 is a lens system having a basic composition consisting of a converging lens group, a diverging lens group and another converging lens group which are arranged consecutively from the object side, and designed for focusing by shifting the diverging lens groups integrally. In case of this lens system, variation of aberrations cannot be suppressed sufficiently though it permits suppressing variation of aberrations to a certain degree by selecting a proper combination of lens shapes. Therefore, it is obliged to sacrifice more or less the performance for photographing an object at infinite distance used frequently in practice in order to maintain satisfactory aberrations in photographing an object at a short distance, especially image plane characteristic. The term "image plane characteristic" used above has the meaning described below:
Optimum offaxial image plane is determined mainly by astigmatism (curvature of field), whereas optimum paraxial image plane is determined by spherical aberration. Therefore, an image plane which is satisfactory from the center to marginal portions can be obtained by correcting aberrations so as to coincide these two image planes with each other. However, it is practically required to consider also influences due to coma and so on. For example, it is dependent on conditions of coma and so on whether the offaxial image plane should desirably be on the positive or negative side of the paraxial image plane when the optimum offaxial image plane is not coincident with the optimum paraxial image plane. It is therefore desirable to coincide image planes as precisely with each other as possible based on overall consideration of conditions of coma and so on. Improving "image plane characteristic" means coinciding the image planes precisely with each other as described above.
In case of the prior example No. 3, variations of spherical aberration and paraxial chromatic aberration cannot be minimized sufficiently though they can be minimized by selecting refractive indices and Abbe's numbers of the lens components comprised in the focusing lens groups and other parameters. Adjustment of the airspaces for correcting aberrations is effective to suppress variation of spherical aberration but does not serve for correcting astigmatism.
As is understood from the foregoing descriptions, the lens systems designed for focusing by displacing one or two movable lens components (lens system comprising 0 or 1 airspace to be adjusted for correcting aberrations) can suppress marginal amount or zonal amount of spherical aberration, but hardly permits maintaining satisfactory image plane characteristic for all objects located from infinite to short distances. As a result, the operability for focusing on an object at a short distance corresponding to a magnification of 1/10 is nearly insignificant.